The present invention is related in general to the field of semiconductor devices and processes and more specifically to the method for fabricating photolithographically defined encapsulated organic light-emitting diodes.
Commercial light emitting diodes (LEDs) typically constitute a p-n junction of inorganic, doped semiconducting materials such as gallium arsenide (GaAs) or aluminum gallium arsenide (AlGaAs). At these junctions between the doped layers, recombination of electrons and holes results in interband emission of light.
In contrast, organic materials are difficult to dope. P-N junctions are not stable and light emitting diodes are therefore designed on p-i-n structures, where the emissive layer in which the charge recombination occurs is nominally intrinsic. Due to the softness of the organic lattice, carriers tend to be polaronic in nature and recombine to form relatively localized molecular excited states (excitons), which then lead to luminescence by the organic material, providing organic light-emitting diodes (OLEDs).
Recently, OLEDs have drawn much attention, especially for emissive display applications. Since OLEDs can be fabricated on any smooth surface, such as silicon wafers, and at low ( less than 100xc2x0 C.) temperatures, they are also very promising for many optoelectronic applications. Electroluminescent devices have been constructed using multi-layer organic films. Basic structure and working are described in xe2x80x9cElectroluminescence of Doped Organic Thin Filmsxe2x80x9d (J. Appl. Phys., vol. 65, pp. 3610-3616, 1989) by C. W. Tang, S. A. VanSlyke, and C. H. Chen. The review article xe2x80x9cStatus of and Prospects for Organic Electroluminescencexe2x80x9d (J. Materials Res., vol. 11, pp. 3174-3187. December 1996, by L. J. Rothberg and A. J. Lovinger) describes various OLED device structures in the form of stacks of thin layers with carrier injection and transverse current flow. For example, the stack may be a transparent substrate (for instance, glass), a transparent anode (for instance, indium-tin oxide, ITO) , a hole transport layer (for instance, TPD), an emissive layer which also is an electron transport layer and in which electron-hole recombination and luminescence occur (for instance, Alq3), and a cathode (a metal with low work function, for instance, magnesium or a magnesium-containing alloy such as Mg:Ag). xe2x80x9cTPDxe2x80x9d is N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(3-methylphenyl)1,1xe2x80x2biphenyl-4,4xe2x80x2diamine. xe2x80x9cAlq3xe2x80x9d is tris(8-hydroxy) quinoline aluminum.
The schematic energy level diagram exhibits discontinuities between the emitter and the hole transport layer. The discontinuity is greater for electron transport to the hole transport layer than the discontinuity in the opposite direction; consequently, holes from the hole transport layer inject into the emitter and recombine with electrons to form excitons, which in turn excite the emitter to luminesce.
A different approach using siloxane self-assembly techniques, has been described in U.S. Pat. No. 5,834,100, issued on Nov. 10, 1998 (Marks et al., xe2x80x9cOrganic Light-Emitting Diodes and Method for Assembly and Emission Controlxe2x80x9d).
In addition to the OLEDs, many related devices such as organic laser diodes, photodetectors, etc. may be realized using organic semiconductors. For many applications such as on-chip interconnects, laser diodes are preferred over LEDs. Laser action has been demonstrated in polymeric organic films, but only by employing optical pumping (for instance, xe2x80x9cLaser Emission from Solutions and Films Containing Semiconducting Polymer and Titanium Dioxide Nanocystalsxe2x80x9d, Chem. Phys. Lett., vol. 256, pp. 424-430, 1996, by F. Hide, B. J. Schwartz, M. A. Diaz-Garcia, and A. J. Heeger; xe2x80x9cLasing from Conjugated-Polymer Microcavitiesxe2x80x9d, Nature, vol. 382, pp. 695-697, by N. Tessler, G. J. Denton, and R. H. Friend; xe2x80x9cSemiconducting Polymers: a New Class of Solid-State Laser Materialsxe2x80x9d, Science, vol. 273, pp. 1833-1836, 1996, by F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Andersson, Q. Pei, and A. J. Heeger). Inadequate charge injection is the main roadblock in achieving an organic-based solid-state laser from electrically pumped organic films.
In their paper xe2x80x9cEnhanced Electron Injection in Organic Electroluminescence Devices using an Al/LiF Electrodexe2x80x9d (Appl. Phys. Lett., vol. 70, pp.152-154, 1997), L. S. Hung, C. W. Tang, and M. G. Mason disclose the beneficial effects of inserting an inorganic dielectric layer (LiF, thin enough for electron tunneling, 0.5 to 1.0 nm) between the metal cathode (Al) and organic material.
The energy bands of Alq3 are bent downwards by the contact with LiF, thus substantially lowering the electronic barrier height of the Alq3-Al interfaces and enhancing the electron injection. The operating voltage is reduced and cathode metals of higher work function can be used. Further, the devices employ a thin (15 nm) buffer layer at the anode (ITO), comprised of CuPc (copper phthalocyanine). The hole transport layer is NPB (N,Nxe2x80x2-bis(1-naphthyl)-N,Nxe2x80x2diphenyl-1,1xe2x80x2-biphenyl-4,4xe2x80x2-diamine). Alq3 is the emissive as well as electron transport layer.
Methods for fabrication and characterization (such as film thickness, and light intensity and wavelength) have been described in xe2x80x9cCharacterization of Organic Thin Films for OLEDs using Spectroscopic Ellipsometryxe2x80x9d (F. G. Celii, T. B. Harton, and O. F. Phillips, J. Electronic Materials, vol. 26, pp. 366-371, 1997). The organic materials may be amorphous or polycrystalline discrete molecular, or may be polymeric. Polymer layers differ from discrete molecular layers because they are typically not fabricated by vacuum vapor deposition, but rather by spin coating from an appropriate solvent. The polymeric layers may also be deposited (either by vapor deposition or by spin coating) as pre-polymer layers and then converted either thermally or photochemically to the active form. Spin coating, spin casting, or melt techniques have the advantage of large area coverage and low fabrication cost.
In U.S. patent application Ser. No. 09/156,166, filed on Sep. 17 1998 (Celii et al., xe2x80x9cOrganic Light Emitting Diodesxe2x80x9d), to which the present invention is related, an OLED is provided with dielectric barriers at both the anode-organic and cathode-organic interfaces. Increased carrier injection efficiencies and increased overall OLED efficiency plus lower voltage operation are thus enabled.
One of the major difficulties in fabricating OLEDs and organic laser diodes (OLDs) is that many solvents used for cleaning or photolithography dissolve the organic layer of the OLEDs and OLDs. As a result, OLEDs are currently fabricated by using shadow masks. Use of shadow masks may be acceptable for certain applications, but the majority of applications, especially those requiring small OLEDs, will require a fabrication process based on photolithography. In addition, since exposure of the organic layer to moisture (or oxygen) may degrade the light-emitting characteristics of the organic material, the organic layer needs to be encapsulated. Although an encapsulation process by reactive ion etching of the organic layer with an aluminum mask layer has been reported in the literature (C. C. Wu, J. C. Sturm, R. A. Register, and M. E. Thompson, xe2x80x9cIntegrated Three-Color Organic Light-Emitting Devicesxe2x80x9d, Appl. Phys. Lett., vol. 69, pp. 3117-3119, 1996), the mask layer was patterned by a shadow mask, not by a photolithography process.
An urgent need has therefore arisen to conceive structure and fabrication methods of electrically pumped organic laser diodes based on photolithography techniques suitable for miniaturization and high process yield. Preferably, this concept should be based on fundamental design solutions flexible enough to be applied for different diode and laser product families and a wide spectrum of material and assembly variations. Manufacturing should be low cost and the devices stable and reliable. Preferably, the innovations should be accomplished using established fabrication techniques and the installed equipment base.
According to the present invention, a method of photolithographically patterning an organic semiconductor device is provided, comprising the steps of protecting the organic layer of the device by depositing a metal layer thereon, depositing and patterning a photoresist layer on said metal layer, and selectively etching the exposed areas to pattern said metal layer and said organic layer. Specifically, the disclosed method provides the photolithographic fabrication of organic light emitting diodes (OLEDs) and organic lasers diodes (OLDs).
It is an aspect of the present invention to form the metal/organic interface before the surface of the organic layer is exposed to the ambient or moisture.
Another aspect of the invention is to fabricate OLEDs whose organic layer is encapsulated.
Another aspect of the invention is produce a plurality of OLEDs concurrently.
Another aspect of the invention is to provide OLED fabrication on a transparent substrate or a semiconductor substrate having an integrated circuit.
In the first embodiment of the invention, the photolithographic patterning of the OLEDs is achieved by protecting the organic layer with the cathode metal layer (xe2x80x9ctopxe2x80x9d electrode metal layer). Preferably, the top metal layer is aluminum. The sequence of process steps is as follows:
1xe2x80x94Forming and patterning the anode (xe2x80x9cbottomxe2x80x9d electrode) layer;
2xe2x80x94Depositing and patterning the insulating layer between the bottom and top electrode;
3xe2x80x94Depositing the organic layer and (immediately thereafter) the top electrode layer;
4xe2x80x94Spin coating and patterning a photoresist layer for defining OLED areas;
5xe2x80x94Removing the top electrode and organic layers in the unmasked areas;
6xe2x80x94Removing the photoresist from the remaining top electrode areas;
7xe2x80x94Depositing the encapsulation layer;
8xe2x80x94Spin coating and patterning a photoresist layer for defining the contact and encapsulation areas;
9xe2x80x94Removing the encapsulation layer in the unmasked areas;
10xe2x80x94Removing the photoresist from the remaining encapsulation layer; and
11xe2x80x94Depositing a protective overcoat layer.
In the second embodiment of the invention, the remaining photoresist is not removed and the encapsulating layer deposited next can make contact to the top electrode layer from the side.
Numerous variations of individual process steps are described. Electrical and optical parameters are employed to demonstrate that no degradation of the OLED characteristics are observed. By way of example, when ion milling is demonstrated as a method for patterning the layers, the need for temperature control is emphasized.
The technical advances represented by the invention, as well as the aspects thereof, will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims.